1. Field of the Invention
The present invention relates to a print head. More particularly, the present invention relates to a defect detection device for detecting defects such as cracks, adhesion failures, etc., in a print head, and a method of detecting defects in a print head.
2. Description of the Related Art
In general, an inkjet printer is a device for printing an image of a predetermined color by ejecting droplets of ink in a predetermined position on a print medium, e.g., a sheet of paper. There are two common types of ink ejection used in inkjet printers. One type is a bubble jet type, wherein an electro-thermal transducer generates bubbles in ink using a heat source, and ink is ejected by the force of the bubbles. The other type is a piezoelectric type, wherein an electro-mechanical transducer ejects ink by changing a volume of an ink chamber due to flexure of a piezoelectric body adjacent to the ink chamber.
FIG. 1 illustrates a conventional piezoelectric inkjet print head, and FIG. 2 illustrates details of a part of the print head of FIG. 1. Referring to FIGS. 1 and 2, a piezoelectric type inkjet print head may include piezoelectric actuators 20, an upper plate 30, ink chambers 40, a middle plate 50, and a lower plate 60. The actuators 20 may be provided on the upper plate 30, and may be structured so as to have thin piezoelectric plates with electrodes stacked thereon to apply a voltage to the piezoelectric plates. The actuators 20 may flex the upper plate 30, i.e., the upper plate 30 may be elastically deformed by the actuators 20 so as to change the volumes of the respective ink chambers 40. The ink chambers 40 may be filled with ink, which is ejected by driving of the actuators 20. Driving the actuators 20 generates a pressure change in the respective ink chambers 40, which causes ink to be ejected from, or, in another part of the cycle, drawn into, the ink chambers 40 because their volume is changed by driving the actuators 20. Passages (not shown) for ejecting ink may be provided in the middle plate 50. Nozzles (not shown) may be provided in the lower plate 60.
A conventional piezoelectric type of an inkjet print head having the above-described structure may be operated as follows. Note that, for clarity, the operation of only a single ink chamber will be described, although, of course, the print head may have many such chambers. Driving the actuator 20 with a first voltage, i.e., a voltage of a first polarity, causes it to flex, which, in turn, causes the upper plate 30 to deform, which, in turn, decreases the volume of the ink chamber 40. Ink inside the ink chamber 40 is ejected to the outside through nozzles of the lower plate 60 by a pressure change caused by the decreased volume of the ink chamber 40. Thereafter, driving the actuator 20 with a voltage of a second polarity causes the upper plate 30 to return to its original shape, increasing the volume of the ink chamber 40. This causes ink to be drawn into the ink chamber 40, due to a pressure change resulting from the increased volume of the ink chambers 40.
The conventional piezoelectric inkjet print head may develop cracks where the upper plate 30 contacts the actuator 20, as indicated by the circled contact region 70 in FIG. 2. In particular, the upper plate 30 is relatively thin over the ink chambers 40 in the contact region 70. Therefore, there is a greater likelihood that a crack will occur in the contact region 70 as compared to other regions.
Another issue affecting the conventional piezoelectric inkjet print head is that adhesion between the upper plate 30 and the middle plate 50 may be poor. Then, as shown in FIG. 2, a separation or aperture may occur in the adhesion region 80 between the upper plate 30 and the middle plate 50. If such an aperture occurs, ink from the ink chambers 40 may leak into the aperture, with detrimental effects on the ability of the print head to correctly eject ink, depending as it does on a pressure change in the ink chamber 40.